This invention relates to a method of gating an electron tube in such a way that a pseudo signal will not appear on its phosphor screen during retracing which follows the main sweeping of photoelectrons emitted from its photocathode. The invention also relates to an electron tube, such as a streak tube in a streak camera, that is operated by said method.
A typical configuration of the streak tube is shown in FIG. 9 and it comprises a photocathode 2 on which the incident light from an input optical system 1 is imaged, deflection plates 3 with which the photoelectrons emitted from the photocathode 2 upon photoelectric conversion are deflected by an applied sweep voltage, a phosphor screen 4 on which the time-dependent change in intensity of the incident light is obtained as the change in brightness with respect to the position of the screen 4, and a microchannel plate (hereinafter abbreviated as "MCP") which is positioned in front of the phosphor screen 4 for multiplying the photoelectrons before their incidence on the screen 4. Shown by numerals 6 and 7 in FIG. 9 are an accelerating electrode and a focusing electrode, respectively, which are arranged between the photocathode 2 and the deflection plates 3 in the order written.
The operation of the streak tube shown in FIG. 9 requires repeated sweeping of the travelling photoelectrons by means of the deflection plates 3, with each main sweep being followed by a retrace. If light is incident on the photocathode 2 during the retracing, it will appear as a pseudo signal on the phosphor screen 4 and the resulting overlap with a true signal obtained during the main sweep introduces difficulty in achieving correct measurements. In order to prevent the generation of such pseudo signals, the photoelectrons that are emitted during retracing have been gated by various methods.
One of such conventional methods is illustrated in FIGS. 10A and 10B for the streak tube shown in FIG. 9. According to this method, when a sweep voltage of FIG. 10A is applied, the occurrence of pseudo signals is prevented by reducing a MCP voltage during the retrace interval as shown in FIG. 10B.
Another method suppresses the emission of photoelectrons by raising the voltage of the photocathode 2 to become equal to that of the accelerating electrode 6.
In a sampling streak camera, shifting electrodes are positioned at a right angle to the main deflection plates and, during retracing, a voltage is applied to said shifting electrodes to bring the travelling photoelectrons out of the field of view.
In a synchroscan streak camera in which a sine wave is applied to the main deflection plates, the photoelectrons emitted during retracing are brought out of the field of view by performing elliptical sweep in which a phase-shifted sine wave is applied to shifting electrodes positioned at a right angle to the main deflection plates.
In order to improve the temporal resolution of streak tubes, it has become popular to use the travelling-wave deflection plates which allow the deflection voltage to propagate at an equal speed to the travelling electrons in the electron tube. The travelling-wave deflection plates used in such streak tubes are designed to have a low characteristic impedance and are driven with a high-voltage, high-speed and low-output-impedance drive circuit. If sweep is done by the combination of such deflection plates and drive circuit, the low impedance of the system will cause retracing to start only a few nanoseconds after each main sweep. Further, jitter is likely to occur between the main sweep timing and the timing of gating in retracing.
Because of these problems, it is difficult to achieve correctly timed retracing with the conventional circuitry. Furthermore, pseudo signals will appear during retracing unless the rise time of gating is shortened to a few nanoseconds, but this has involved considerable difficulty.